Generally, September is one of the easier times to book on shorter notice. However, depending on where you want to stay, villa type & specific dates, you could run into some trouble. If you are flexible with resort, dates and villa size/type, you will virtually always find space on short notice for a September stay.
In general:
Keep in mind that the last weekend of September is the beginning of the Food & Wine Festival at EPCOT. This is a very busy time for the EPCOT resorts & BLT (monorail access to EPCOT). You may run into problems if your dates include this weekend.
Less expensive villa types are almost always filled shortly after the 7 month window opens (if not before).
Good luck!